Service Tranformer Puzzle

[cslater]

So I think this will be easy for some of you. I feel like it should be easy for me but I can't quite wrap my head around it.

I'm looking at a building that is served by three power conductors from three separate pole mounted transformers. One of the main switchboards has only two conductors going to it, and the other has three. Clearly single phase and three phase, but I want to understand what the voltages are.

The pole transformers are set up like this:


Transformer 1 - Conductor out and connected to XF 3 / Ununsed / Connected to XF 2
Transformer 2 - Connected to XF 1 / Ground / Conductor out and connected to XF 3
Tranformer 3 - Connected to XF2 / Unused / Connected XF3

In my mind this would give two conductors that are 120V to ground and 180 degrees apart - so single phase 240. How does that third conductor make the other panel three phase? It seems like the third phase couldn't possibly be 120 degrees out...

Am I missing something?

Thanks!

 

[jim dungar]

You have a center tapped delta arrangement. The ANSI method for describing this would be 240/120V [3-phase 4-wire].
A-B=240V
B-C=240V
C-A=240V
A-N=120V
N-C=120V

B-N=208V

The B-N voltage is high, so this connection is often called by names like; wild-leg, high-leg and even xmas-tree.
Commonly this connection only uses two transformers (an open delta), so you are one of the lucky ones if you have three (a closed delta)

 

[cslater]

That makes sense. Thank you so much Jim!

 

[cslater]

Follow up question now that I've had some time to digest this...

If I want to pull off a panel for 120V circuits, and the upstream panel doesn't have a neutral, how do I do that?

I'm thinking you would just pull off the two phases that are 240V apart and feed them to a panel. But where to get a neutral?

The other thing I would think is that you would put in a delta/wye transformer, but the neutral is not in the right place, so it seems like you would need a special transformer in this case?

 

[Smart $]

Follow up question now that I've had some time to digest this...

If I want to pull off a panel for 120V circuits, and the upstream panel doesn't have a neutral, how do I do that?

I'm thinking you would just pull off the two phases that are 240V apart and feed them to a panel. But where to get a neutral?

The other thing I would think is that you would put in a delta/wye transformer, but the neutral is not in the right place, so it seems like you would need a special transformer in this case?

I agree with Jim's earlier assessment. That said, it's a requirement for the grounded conductor (the so-called neutral in this case) to be brought to the service disconnect. Check there...

Please clarify what you mean by "upstream panel" as it relates to the service disconnecting means and the grounded conductor...

 

[gar]

131114-2135 EST

cslater:

You describe transformer 2 as two hots and a ground. Your ground is the center tap of a 240 V single phase transformer. This terminal will be grounded at the pole with a ground rod. If 120-0-120 is provided at your main panel, then what you call ground should have a neutral conductor running from the pole to the main panel. This neutral conductor will be grounded again at the main panel.

But this would imply 4 wires from the pole transformers, but you said there were only three wires. This would mean you only have a 240 delta at the main panel that has a ground reference off of a center tap back at the pole. Is it really true there are only three wires from the pole?

If only three wires, then you will need a transformer from 240 V to a 240 V center tapped secondary. Or get the power company to provide a neutral from the pole to your main panel.

.

 

[cslater]

There is a 240V 3 wire meter panel (service entry) and that feeds a 240V delta panel used for A/C units. I'm calling that one the "upstream" panel.

So if I want to serve 120V loads from that A/C panel, it seems like I could just feed a single phase 240V panel from the two lower legs, using the ground in that panel for my neutral. I just don't know if that's a code violation and/or will create too much imbalance.

The other option would be to put in a delta/wye transformer, but I can't understand how that would work since ground is not in the middle of the triangle like in a normal delta load but is down at the bottom of the triangle between the two 120V legs. Do they make a special transformer for that?

 

[jim dungar]

There is a 240V 3 wire meter panel (service entry) and that feeds a 240V delta panel used for A/C units. I'm calling that one the "upstream" panel.

So if I want to serve 120V loads from that A/C panel, it seems like I could just feed a single phase 240V panel from the two lower legs, using the ground in that panel for my neutral. I just don't know if that's a code violation and/or will create too much imbalance.

The other option would be to put in a delta/wye transformer, but I can't understand how that would work since ground is not in the middle of the triangle like in a normal delta load but is down at the bottom of the triangle between the two 120V legs. Do they make a special transformer for that?

At the service entrance equipment the ground and neutral will be the same point. Actually the neutral comes from the transformer and the ground comes from the grounding electrode, but they join together at the service.


You would only need a delta-wye transformer if you want three phase 208Y/120 for your loads. If you only need 120/240, then a single phase unit would work.

A, non-utility, transformer primary never connects to the primary side neutral, so it does not care where it is. The transformer primary simply needs to be rated for the L-L voltage.

 

[Smart $]

There is a 240V 3 wire meter panel (service entry) and that feeds a 240V delta panel used for A/C units. I'm calling that one the "upstream" panel.

So if I want to serve 120V loads from that A/C panel, it seems like I could just feed a single phase 240V panel from the two lower legs, using the ground in that panel for my neutral. I just don't know if that's a code violation and/or will create too much imbalance.

...

The ground connection of Transformer 2 should be brought to the service disconnecting means... the 240V 3-wire meter panel. If the A/C panel has a neutral bar, should simply be a matter of bringing a grounded "neutral" from the meter panel to it for 120/240V 1? 3W, other compliance issues not withstanding.

 

[gar]

131115-0624 EST

cslater:

How many large current carrying conductors come to the meter from the pole transformers? There should be 4. There should be 3 hot lines, and one neutral from the center tapped transformer secondary. Your original post is not clear on this.

Transformer 1 - Conductor out and connected to XF 3 / Unused / Connected to XF 2
Transformer 2 - Connected to XF 1 / Ground / Conductor out and connected to XF 3
Transformer 3 - Connected to XF2 / Unused / Connected XF3

You only describe two out conductors, and a ground. Also you have Transformer 3 connected to XF3. I assume this should be XF1.

The description lacks an out from the XF2-XF1 connection.

My analysis from your description is:

1. One end of XF1 is connected to one end of XF3. Call this point A, and would be called the wild leg based on my assumption of your circuit. From this point there is an out wire, and the center taps of XF1 and XF3 are not used.

2. The other ends of XF1 and XF3 connect to the outer ends of XF2. From the connection of XF2 to XF3 there is an out wire. Call this B.

3. The connection of XF1 to XF2 goes nowhere. Call this C.

4. The center tap of XF2 goes to ground (earth). No indication that this is an out wire. Call it N.

5. Between ground (earth) and the out terminal of XF2-XF3 is 120 V.

6. Between ground (earth) and the out terminal of XF1-XF3 is 208 V.

Is this what you have?

.

 

[kwired]

Are any conductors at the service gear bonded to the enclosures or connected to a grounding electrode system?

Sounds like from your description there is a 4 wire grounded system, but there may be no grounded conductor brought to the service equipment

 

[cslater]

OK - here is what it looks like



So they are bringing the three phase conductors down the conduit and then tying the ground to the conduit. I assume that ground in the panel is bonded to the conduit as well and that there is building ground at the service entrance panel. I will need to verify.

Based on that, I think what I'm hearing is that I can get single phase power by pulling a breaker from the two 120V legs and connecting the grounds or get three phase by using a delta-wye isolation transformer to get 208Y.

Just to clarify - since I need to pick up a neutral and the only way to do that is at a transformer - if I want to go with 240V single phase on the subpanel, would I need to go through a single phase isolation transformer?

 

[kwired]

OK - here is what it looks like

View attachment 9345

So they are bringing the three phase conductors down the conduit and then tying the ground to the conduit. I assume that ground in the panel is bonded to the conduit as well and that there is building ground at the service entrance panel. I will need to verify.

Based on that, I think what I'm hearing is that I can get single phase power by pulling a breaker from the two 120V legs and connecting the grounds or get three phase by using a delta-wye isolation transformer to get 208Y.

Just to clarify - since I need to pick up a neutral and the only way to do that is at a transformer - if I want to go with 240V single phase on the subpanel, would I need to go through a single phase isolation transformer?

Can you get another conductor in the raceway instead of using the raceway as the grounded conductor?

Are all three phases isolated from equipment grounding conductors, raceways, enclosures, etc?

Kind of sounds like someone either didn't know what they were doing, or the system once was corner grounded or ungrounded and whoever changed it did not know what they were doing.

 

[Smart $]

Can you get another conductor in the raceway instead of using the raceway as the grounded conductor?

Was using conduit ever permitted as the grounded conductor? I realize your suspicions (not quoted) are likely so... but it is currently non-compliant, regardless of it not being used as a load current conductor. Not really in question whether he can get in the fourth conductor in... it has to be put in the raceway (or its replacement, if needed).

 

[kwired]

Was using conduit ever permitted as the grounded conductor? I realize your suspicions (not quoted) are likely so... but it is currently non-compliant, regardless of it not being used as a load current conductor. Not really in question whether he can get in the fourth conductor in... it has to be put in the raceway (or its replacement, if needed).

Maybe I should have worded it differently, but that is basically what I was meaning.